In recent years, electromechanical transducers produced by a micromachining process have been researched actively. In particular, capacitive electromechanical transducers called CMUT (Capacitive Micromachined Ultrasonic Transducers) have attracted attention, because they can transmit and receive ultrasonic waves with a lightweight vibrating membrane, and can more easily obtain wide band characteristics even in the liquid and air than piezoelectric electromechanical transducers that have been used hitherto.
The structure of a CMUT will be described below. A CMUT includes a plurality of elements arranged in an array in a one-dimensional or two-dimensional direction. These elements serve to transmit or receive ultrasonic waves.
The structure of an element in the CMUT will be described below. As shown in FIG. 11A, an element 301 of a CMUT includes a plurality of cells 311. By simultaneously applying driving voltage signals of the cells 311 in the element 301, ultrasonic waves are output from the element 301. Further, ultrasonic detection signals received by the cells 311 in the element 301 are added by upper electrodes 315 and lower electrodes (not shown) that are common to the cells 311, and the sum of the signals serves as an ultrasonic detection signal received by the element 301. Lines 307 electrically connect the upper electrodes 315 of the cells 311.
U.S. Pat. No. 6,958,255 discloses an example of a CMUT having such an element structure. In the CMUT disclosed in U.S. Pat. No. 6,958,255, a substrate through line 304 is provided in a support substrate 305, as shown in FIG. 11B. The substrate through line 304 electrically connects a circuit board 303 and a lower electrode 316. In the circuit board 303, driving voltage signals are generated to output an ultrasonic wave from an element, and signal processing, such as amplification and delay addition, is conducted on an ultrasonic signal generated by an ultrasonic wave received by the element.
The structure and operation principle of the cell of the CMUT will be described below. As shown in FIG. 12A, a cell 311 of the CMUT includes a membrane 312, an insulating layer 313, a cavity 314, an upper electrode 315, and a lower electrode 316. The upper electrode 315 and the lower electrode 316 constitute a capacitor, and a bias voltage is applied therebetween by a bias voltage source 317. For transmission of an ultrasonic wave, a driving voltage signal having a proper waveform is applied between the upper and lower electrodes 315 and 316 by a driving voltage signal source 318, whereby the membrane 312 vibrates to generate an ultrasonic wave in accordance with the driving voltage signal, as shown in FIG. 12A. Conversely, for receiving, the membrane 312 is vibrated by an ultrasonic wave reaching the CMUT, whereby an electrostatic capacitance between the upper and lower electrodes 315 and 316 changes and a current signal is generated in accordance with the ultrasonic wave, as shown in FIG. 12B. By detecting this current signal with a current detector 319, the received ultrasonic wave can be detected.